The present invention relates to an optical multiplexing/demultiplexing module having a function of synthesizing polarized waves and a function of demultiplexing a wave into polarized waves.
Recently, optical multiplexing/demultiplexing modules are widely used in optical communication systems and in the field of optical measurement and the like. In general, an optical multiplexing/demultiplexing module functions as an optical multiplexer and as an optical demultilexer. Optical multiplexing/demultiplexing modules include optical multiplexers based on a method in which a plurality of incident beams having different wavelengths are multiplexed (or synthesized) to emit and obtain a multiplexed beam. By using such an optical multiplexing/demultiplexing module based on the wavelength multiplex/demultiplex method, a plurality of lights having different wavelengths can be transmitted as a multiplexed signal beam. Further, when the optical multiplexing/demultiplexing module is used as an optical demultiplexer, a multiplexed signal beam is demultiplexed into signal beams having different wavelengths.
Optical multiplexing/demultiplexing modules include optical multiplexers based on a method in which two linearly polarized incident beams having planes of polarization orthogonal to each other are multiplexed to emit and obtain a single multiplexed beam and in which the multiplexed beam is demultiplexed into two linearly polarized waves. An optical multiplexing/demultiplexing module based on the linearly polarized beam multiplexing/demultiplexing method is also referred to as xe2x80x9cpolarized beam combiner/splitter. For example, an optical multiplexing/demultiplexing module based on the linearly polarized beam multiplexing/demultiplexing method is used for synthesizing laser beams from a semiconductor laser or the like to provide higher optical power, as described later.
The plane of polarization implies a plane having a direction in which a linearly polarized beam is propagated and a direction in which an electric vector (electric field) oscillates.
Although the semiconductor lasers are widely used as light sources in the field of optical communication, it is presently difficult to provide a semiconductor laser having high power that can satisfy requirements in the field. Further, it is known that when a semiconductor laser oscillates laser beams that are linearly polarized beams, improved laser beam optical power can be achieved by synthesizing two laser beams having planes of polarization orthogonal to each other.
Conventional optical multiplexing/demultiplexing modules based on the linearly polarized beam multiplexing/demultiplexing method include devices utilizing a uniaxial birefringent crystal and devices utilizing a polarization beam splitter.
FIG. 6 is an illustration of an example of an optical multiplexing/demultiplexing module based on the linearly polarized beam multiplexing/demultiplexing method. The optical multiplexing/demultiplexing module is an example of an optical multiplexing/demultiplexing module in which a uniaxial birefringent crystal is used to multiplex linearly polarized beams having planes of polarization orthogonal to each other. In FIG. 6, a uniaxial birefringent crystal 78 multiplexes a first linearly polarized beam 71 which is emitted by a first polarization-maintaining optical fiber 70 and collimated by a first lens 72 and a second linearly polarized beam 73 which is emitted by a second polarization-maintaining optical fiber 74 and collimated by a second lens 76.
As shown in FIG. 6, the polarization-maintaining optical fibers 70 and 74 are provided substantially in parallel with each other at an interval, and the first linearly polarized beam 71 and second linearly polarized beam 73 enter the uniaxial birefringent crystal 78 with their optical axes substantially in parallel with each other. When the first linearly polarized beam 71 and second linearly polarized beam 73 enter an entrance surface 77 of the uniaxial birefringent crystal 78, the linearly polarized beams 71 and 73 are multiplexed at an exit surface 79 of the uniaxial birefringent crystal 78. The multiplexed beam 75 is converged by a lens 80 to enter an optical fiber 82.
An optical multiplexing/demultiplexing module according to the invention comprises:
a first optical input section for inputting a first linearly polarized beam;
a second optical input section provided at an interval from the first optical input section for inputting a second linearly polarized beam having a plane of polarization orthogonal to that of the first linearly polarized beam;
a uniaxial birefringent crystal for multiplexing the first linearly polarized beam and the second linearly polarized beam:
an optical output section for outputting a multiplexed beam multiplexed by the uniaxial birefringent crystal; and
an optical path converting member for converting at least either of the optical path of the first linearly polarized light propagating from the first input section and the optical path of the second linearly polarized beam propagating from the second input section when they enter the uniaxial birefringent crystal,
wherein the optical path converting member performs the optical path conversion such that the distance between the points of entrance of the first linearly polarized beam and second linearly polarized beam entering the entrance surface of the uniaxial birefringent crystal becomes smaller than the distance between the first optical input section and second optical input section.